1. Field of the Disclosure
The present disclosure relates to a fiber laser system configured with one or more multimode fibers each of which is configured so that the coupling between a fundamental mode and one or more high-order modes propagating along the multimode fiber is substantially minimized.
2. Prior Art Discussion
Light propagating along a optical fiber may have one or more propagation paths commonly referred to as modes. An optical fiber may support a single, fundamental mode or more than one mode depending on physical and geometrical characteristics of the fiber. As a consequence, a fiber configured to support only a fundamental mode is referred to as a single-mode (SM) fiber, whereas a fiber guiding more than one mode is a multimode (MM) fiber.
As known, in a MM active fiber, i.e., the fiber provided with a gain medium doped with rare-earth elements, the modes are amplified. In addition, the modes tend to couple to one another. The coupling effect is particularly pronounced when the gain coefficient of HOM is at most equal to or less than the gain coefficient of the fundamental mode, which may occur when the doped profile is substantially close to the intensity of the fundamental mode. The mechanism of mode coupling has been studied for a long time and is rather complicated. In very broad terms, the periodic modulation of light intensity along the fiber leads to the modulation of the refraction index, which in turn may create the coupling of modes. In large degree, the mode coupling is a result of the change of the medium's refractive index due to excitation of electrons which leave a ground energy level for higher ones.
Regardless of the complexity of the mode-coupling theory, this concept can be easily understood if described in terms of quality of light. As the modes couple to one another, the total power of single mode light beam coupled into the MM fiber will be distributed among two or more modes, one of which is a fundamental mode carrying the major part of the light power. If a MM fiber is configured to radiate a high quality beam, all it means that the total power should be concentrated in the fundamental mode. Accordingly, as the power is drawn from the fundamental mode, which happens when at least one higher-order mode is coupled to the fundamental mode, the “useful” power reduces. Thus, the desired power of the light beam radiated from a MM optical fiber can be achieved when the coupling between the fundamental and one or more high-order modes is substantially eliminated.
One may ask why bother with MM fibers if SM fibers, which are not associated with a cross-coupling phenomenon, can be used. Many applications of fiber laser systems require high pump powers. High pump powers, however, cause serious problems associated with the design of fiber systems. For example, with higher powers comes a problem associated with nonlinear effects which, for a few notable exceptions, are highly undesirable. To raise the threshold for nonlinear effects in a fiber, it is desirable to increase the core diameter. In other words, to avoid the presence of nonlinear effects at relatively high pump powers, the effective mode area should be as great as possible. This requirement, however, can be better met by MM fibers. Accordingly, the use of MM fibers, characterized by a relatively large core diameter, becomes practically a norm in high power fiber laser systems. Thus, in some applications of MM fibers requiring a high quality output, the mode-coupling should be suppressed.
A need, therefore, exists for providing a method for minimizing the coupling between fundamental and higher order modes in conventional multimode fibers while concentrating the majority of the total input power in a fundamental mode.
A further need exists for a high power fiber system configured in accordance with the disclosed method and characterized by high quality powerful output light beam.